Resonant cavity excitation system



provide a method and apparatus 2,868,974 RESONANT CAVITY EXCITATION SYSTEM William R. Baker and Quentin A.

Jack Riedel, SanPablo, Califi, assignors to the United States of America as represented by the United States Atomic EnergyCommission Application March 26, 1954, Serial No. 419,124

3 Claims. (Cl. 250-36) Kerns, Berkeley, and

build up to maximum value. Several factors are responsible for such failure of the oscillator circuit and may be generally listed as follows:

( 1) The low-power gain of the available oscillator tubes for small signals;

(2) The large energy storage or flywheel effect of the cavity; and

(3) ?Ion locking which is defined as a pressure sensitive loading of the oscillator.

To overcome the foregoing factors a sub-exciter is utilized in conjunction with the main oscillatorand a Additionally, there is a saving in. power resulting from the fact. that the initial voltage of the cavity is at a noise level of the order of l-l4 volts so that more time is required to increase the voltage from such level to one volt, for example, than from one volt to the required level of several millions of volts. A substantially lowvoltage pre-exciter therefore conserves power require ments.

.It is. therefore an object of the present invention to for exciting a cavity resonator.

Another object of the invention is to provide a method and apparatus for exciting a cavity resonator incrementally.

Afurther object of the invention is to provide an exciter for a large cavity resonator having means for overcoming ion locking.

Still another object of the invention is to provide lowvoltage excitation of a cavity prior to the high-voltage excitation thereof.

Other objects and advantages of the invention will. be apparent in the following description and claims considered together with the accompanying drawing which is a schematic wiring diagram, partially in block form, of the invention.

Referring to the drawing in detail, there is provided a cavity resonator 11 and a conventional radio-frequency oscillator 12 which is capable of delivering millions of volts at the output 13. A coaxial transmission .line 14 :1. rates tt positive polarity and a is suitablyyconnected from the output 113 of the oscillator 12 to the cavityresonator llwhere a coupling loop 16 extends internally of the cavity and terminates the line. To supply a high-voltage operating potential to the radio-frequency oscillator 12, there is provided a conventional power supply 21 having a terminal 22 of grounded terminal 23. Such terminals 22 and 23 are respectively connected topower input terminals 24 and 26 of the radio-frequency oscillator 12. Preferably, the power delivered by the power supply 21 is pulsed.

.A sub-exciter power supply 31 having a terminal 32 at a positive potential and a grounded terminal 33 is provided with a diode tube. 34 having the anode connected to the positive terminal and the cathode connected to the positive terminal22 of the high-voltage power supply 21. The voltage between the terminals 32 and 33 of the sub-exciter powersupply 31 is established at a value less than that of the high-voltage power supply 21, but greater than the value necessary to excite the cavity resonator 11 at the levelwhere ion locking occurs. Since the diode tube 34 is a unidirectional device, the high voltage of the high-voltage power supply 21 .is prevented from feeding back to the sub-exciter power supply 31. Thus, with the foregoing connections the radio-frequency oscillator 12 is supplied at all times with sufficient operating voltage to excite the .cavity resonator 11 above the level of ion locking.

A pickup loop 41 is disposed within the cavity resonator l1 and suitably connected to one endofacoaxial transmission line 42. The input 43 of a limiting amplifier .44 is connected at the other end of the transmission line 42 and such amplifier provides a voltage at the output t6 thereof which is constant in amplitudefor all values of input voltage. Another length of coaxial transmission line 47 is suitably connected from the output 46 of the limiting amplifier 44 to the signal input 48 of a preexciter 51. j

The pre-exciter 51 comprises, in general, a pulsed power supply 52 capable of delivering a value of voltage which is between the values of the other two power supplies El and 31, and two tetr'ode type power amplifier tubes 53 and 54. The input 48 'is connected between the control grid and grounded cathode of the first amplifier tube 53 so as to impress the output voltage of the limiting amplifier 44 upon the control grid. A tank circuit 56, which comprises a variable capacitor 57 and parallelconnected inductance coil 58, is connected at one end directly to the anode of the first tube 53 and at the other end by a dropping resistor 59 to the positive terminal 61 of the pulsed power supply 52. The values of the capacitor 57 and coil 53 are selected so that the tank circuit 56 is resonant at the frequency of the excitation of the cavity resonator 11. The negatively charged terminal 62 of the pulsed power supply 52 is grounded to establish the voltage reference. A resistor 63 .is connected from the suppressor grid of the first tube 53 to the positive terminal 61 of the pulsed power supply 52, and a parallelconnected resistor 66 and capacitor 67 combination is connected from the suppressor grid to ground. To complete the connections of the first amplifier circuit and to preventvoltage variations of the tank circuit 56 from reaching the pulsed power supply52, a by-pass capacitor 69 is connected from the junction of the tank circuit and the dropping resistor 59 to ground.

To inductively couple voltage variations at the tank circuit 56 to the second amplifier tube 54, a pickup coil 71 is disposed adjacent to the inductance coil 58 of the tank circuit and is connected between the control grid and grounded cathode of the tube. A small variable inductor 72 is connected from the anode of the second tube 54 to the positive terminal 61 of the pulsed power supply 52 with a by-pass" capacitor 73 connected from the power supply side of the inductor to ground. It is to be noted that the inductor 72 may be varied to compensate for the effect of the interelectrode capacitances of the second amplifier tube 54. A voltage divider comprising two series-connected resistors 76 and 77 is connected from the positive terminal 61 of the pulsed power supply 52 to ground with the junction between the resistors connected to the suppressor grid of the second amplifier tube 54.

Another section of coaxial transmission line 81 is extended from the pre-exciter 51 to the cavity resonator 11. At the pre-exciter end of such transmission line 81 the central conductor is connected to the anode of the second amplifier tube through a coupling capacitor 82 and the outer conductor is connected to ground. A coupling loop 86 is provided at the cavity resonator end .of the transmission line 81 and is suitably connected thereto. The coupling capacitor 82, while serving to transfer voltage variations at the anode of the second amplifier tube 54 and blocking direct current components, is also selected to have a value such that the capacitive reactance thereof is equal to the inductive reactance due to the coupling loop 86. In such manner the second amp-' lifier tube 54 is working into a purely resistive load. The area of the coupling loop 86 is selected to be substantially large as an aid in exciting the cavity resonator 11. Such fact results in a high value of induced voltage in the coupling loop 86 when full excitation of the cavity resonator is accomplished, and to protect the pre-exciter 51 it is necessary to either withdraw the loop or rotate it to a position of minimum coupling with the field. Either of such procedures may be successfully followed, as well as the suitable insertion of a T -R box in the output circuit of the pre-exciter 51. (For simplicity the elements for such conventional protective means have not been shown on the drawing.)

Consider now the operation of the system assembled as set forth in the foregoing and with the sub-excit'er power supply 31 in operation. Under such condition the radio-frequency oscillator 12 operates to excite the cavity resonator 11 at a low level, for example 150 kv., at the resonant frequency of the cavity. A small value of induced voltage exists at the coupling loop 41 and is transferred by the transmission line 42 to the input 43 of the limiting amplifier 44. A voltage having a constant amplitude and a frequency equal to the resonant frequency of the cavity resonator 11 is coupled from the output 46 of the limiting amplifier 44 to the pre-exciter 51 by the transmission line 47. The latter radio-frequency voltage is impressed at the control grid of the first amplifier tube The pulsed power supply 52 is turned on to generate a positive going square wave of voltage at the time the radio-frequency signal voltage is impressed at the control grid of the first amplifier tube 53. Thus, the first amplifier tube 53 is suitably impressed with operating voltages and conduction occurs in accordance with the control grid voltage. The result is an amplified radio-frequency voltage at the tank circuit 56 which is resonant at the frequency of the resonant frequency of the cavity resonator 11.

Inductive coupling between the coil 58 of the tank circuit 56 and the pickup coil 71 provides a driving radiofrequency voltage having the resonant frequency of the cavity resonator 11 at the control grid of the second amplifier tube 54. Amplification occurs in the circuit of the second amplifier tube 54 and an intermediate value of radio-frequency voltage is coupled by the capacitor 82, transmission line, and coupling loop 86 to the cavity resonator 11 for the excitation thereof.

After the voltage in the cavity resonator 11 has reached maximum value under excitation by the pre-exciter 51,

the high voltage power supply 21 is turned on to impress a high value of voltage upon the radio-frequency oscillator 12. At such time the sub-exciter power supply 31 is protected from the high voltage by the diode 34. Concurrently with the foregoing the pulsed power supply 52 of the pre-exciter 51 is turned off and the loop 86 is rotated to a position of minimum coupling (for example) to protect the pre-exciter. The cavity resonator 11 is then excited to the maximum high voltage value of radio frequency by the radio-frequency oscillator 12.

With the foregoing it has been found that a minimum of difiiculty is encountered in raising the excitation of a cavity resonator to maximum value. A large cavity resonator having dimensions measured in feet was readily excited to millions of volts with a sub-exciter power supply 31 voltage of 2 to 3 kv. and a pulsed power supply voltage of 10 kv. The problem encountered by ion locking, which has been described as the cascade ionization of residual molecules in the cavity, was eliminated.

While the salient features of the present invention have been described in detail with respect to one embodiment, it will be apparent that numerous modifications may be made within the spirit and scope of the invention, and it is therefore not desired to limit the invention to the exact details shown except as defined in the following claims.

What is claimed is:

1. In a system for exciting a cavity resonator, the

combination comprising a cavity resonator having a pre- I determined resonant frequency, a high-frequency oscillator coupled to said cavity resonator and adapted to oscillate at said resonant frequency, means coupled to said oscillator for establishing a base level of excitation at said cavity resonator, a power amplifier having an input and an output, means coupled between said cavity resonator and said input for driving said amplifier in response to excitation of said cavity resonator at said base level, means coupled between said output and said cavity resonator for applying a medium value of voltage to the latter at said resonant frequency to increase the excitation of said resonator to an intermediate level, said amplifier being turned off subsequent to the attainment of the intermediate level of excitation in said resonator, and second means connected to said oscillator for establishing a high value of excitation .at said cavity resonator while simultaneously disabling said first means, said second means being turned on upon the attainment of said intermediate level of excitation in said cavtiy.

2. In a system for exciting a cavity resonator, the combination comprising a cavity resonator having a predetermined resonant frequency, a high-frequency oscillator coupled to said cavity resonator and adapted to oscillate at said resonant frequency, said oscillator having a power input and a radio-frequency output, a sub-exciter power supply coupled to said power input whereby said cavity resonator is excited at a base level, a pre-exciter having an input and an output, means coupled between said cavity resonator and the input of said pre-exciter for driving said pre-exciter in response to excitation of said cavity resonator at said resonant frequency, pulsed power supply means coupled to said pre-exciter to energize same upon excitation of said cavity resonator at said base level, means connected between the output of said preexciter and said resonant cavity for coupling the voltage of the former to the latter, said pre-exciter being adapted to develop a radio-frequency output voltage having a medium value, said pulsed power supply means de-energizing said pre-exciter to disable same subsequent to the development of said radio frequency voltage of medium value in said-cavity resonator, a pulsed power supply connected to the power input of said oscillator for applying a high value of voltage thereto for exciting said resonator to a preselected maximum value, said pulsed power supply tie-energized upon the attainment of said voltage .of medium value in said cavity resonator, and means connected between said pulsed power supply and said sub-exciter power supply to disable the latter during operation of the former.

3. The combination of claim 2 wherein the means coupled between said cavity resonator and the input of said pre-exciter is a limiting amplifier having an output voltage of constant amplitude for all input voltages.

6 References Cited in the file of this patent UNITED STATES PATENTS Varian Jan. 8, 1946 Young Feb. 25, 1947 Hansen Sept. 28, 1948 Jensen Feb. 21, 1950 

